It is a well-known fact that low-frequency sound waves can travel longer distances through water than can high-frequency sound waves. For a long time there has also been a considerable need of powerful low-frequency sound transmitters which are capable of working under water, both from a military point of view and from the point of view of the offshore oil and gas industry. Such transmitters have also been available on the market for quite a long time. A summary of acoustic transmitters for such purposes is given in an article in DEFENSE SYSTEMS REVIEW, November 1984, pages 50-55 entitled "Sonar transducer design incorporates rare earth alloy".
Most acoustic transmitters which are used at present are based on either the piezoelectric effect or on magnetostriction. As is well-known, the piezoelectric effect means that a crystalline substance is subjected to a change in length when an electric voltage is applied to its end surfaces and that a voltage is obtained across its end surfaces when the substance is subjected to a physical deformation, respectively. The magnetostriction means that a magnetic material which is subjected to a change of the magnetic flux suffers a change in length and that an externally caused change in length gives rise to a change in the magnetic flux, respectively. This means that a transmitter which utilizes these effects can also, in principle, be used as a receiver.
A variety of different embodiments of acoustic transmitters exist. In low-frequency applications it is common that they have a cylindrical shape with either a circular or elliptical cross section area.
The greatest problem with this type of transmitter is to achieve a sufficiently great amplitude of the oscillations. To this end either a large transmitter area or a small transmitter area with great amplitude of oscillation would be required.
The introduction of the so-called giant (rare earth) magnetostrictive materials has improved the conditions for obtaining good acoustic transmitters. With such materials as driving elements in the transmitters, amplitude changes may be obtained which may largely amount to 100 times the corresponding changes using piezoelectric materials or using ordinary magnetic materials. Transmitters which utilize these giant magnetostrictive materials have existed on the market for several years.
A frequently occurring embodiment for the actual driving will be described in greater detail starting from a cylindrical transmitter with an elliptical cross section. The cylindrical envelope surface consists of an elastic diaphragm or shell. Inside and parallel to the axis of the cylinder and making contact with the shell are two rods applying pressure to the shell. The cross section area of the rods is symmetrically mirror-inverted in relation to the minor axis and each rod is delimited by that part of the contour of the ellipse which faces the end of the major axis and a chord parallel to the minor axis. Between the rods and making contact with their plane-parallel sides there is arranged an electrically controlled driving element in the form of a driving rod. The longitudinal axis of the driving rod coincides with the major axis of the elliptically formed cross section and lies midway between the end surfaces of the transmitter. In those case where the magnetostrictive effect is utilized, the driving rod consists of a magnetic material, suitably a giant magnetostrictive material, which with a surrounding winding is magnetized to keep pace with the desired frequency of the transmitter. If the piezoelectric effect is to be utilized, the driving rod is made of a piezoelectric material. The driving rod may, of course, consist in its entirety, or in certain parts, of a material with the desired possibilities of changing the length.
The fundamental embodiment of an acoustic transmitter described above may be different as regards the actual details. An acoustic transmitter with a cylindrical shape and with an elliptical cross section area and with driving rods of a giant magnetostrictive material, is disclosed, inter alia, in International Publication No. WO 86/03888, dated July 3, 1986.
As will have been clear from the above, it is desirable to obtain as great changes in amplitude as possible. The choice of the shape of the elliptical cross section area is therefore of great importance. It is to be noted that the ratio between the major axis and the minor axis of the ellipse is often chosen as 2:1. If a certain change of length of the major axis is obtained with the aid of the driving rod, the change of length of the minor axis will be 2-4 times as great, all according to the properties of the shell and the shape of other parts.